Low temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. One of the advantages of LTCC is the ability to integrate passive components, such as capacitors, resistors, inductors, resonators, and filters, in the LTCC via cofiring processes. For LTCC modules with embedded resonator functions targeting high frequency applications, the temperature coefficient of resonant frequency (τf) is a critical parameter. A low τf, preferably close to 0 ppm/° C., is desirable to achieve temperature-stable resonator functions for radio and microwave frequency (RF and MW) applications. With a low τf resonator, the temperature compensation that requires additional mechanical and electrical circuits to achieve temperature stable resonator function can be eliminated. See C. Wang and K. A. Zaki, IEEE MMT-S Intern. Microwave Sump, Digest 3, 1041 (1993); and U.S. Pat. No. 5,302,924 to H. Johnson and A Tunumen. For filter applications, a low τf enables efficient use of bandwidth for maximum communication capacity.
There are multiple approaches to achieve temperature stable resonator functions. One common engineering method to achieve a near zero τf is to form solid solutions of different dielectric ceramics that have opposite τf. The method has been applied to many MW dielectrics for resonator applications as well as LTCC base dielectrics for integration of resonator functions. See W. Wersing, Solids State and Mat. Sci. 1, 715 (1996); R. C. Kell et al., J. Am. Ceram. Soc. 56, 352 (1973); S. Kucheiko et al., J. Am. Ceram. Soc. 79, 2739 (1996); I. M. Reaney, and D. Iddles, J. Am. Ceram. Soc. 89, 2063 (2006); S. Dai et al., J. Am. Ceram. Soc. 85, 828 (2002); H. Jantunen, J. Am. Ceram. Soc. 85, 697 (2002); H. Kapta et al., Jpn. J. Appl. Phv. 31, 3152 (1992); Y. Choi et al., Materials Letters 58, 3102 (2004); and U.S. Pat. No. 6,841,496 to Kim. The other practice is to form a hybrid layered structure incorporating alternating layers with opposite τf, either by sequential sintering or bonding processes. See N. Alford et al., IEE Proc-Sci. Meas. Technol. 147, 269 (2000); and L. Li and X. M. Chen, J. Am. Ceram. Soc. 89, 544 (2006).
Although there are commercial LTCC dielectrics with low τf, (e.g., Hereaus CT2000 with τf<10 ppm/° C.), most of existing LTCC dielectrics possess a τf in the range −50 to −80 ppm/° C. FIG. 1 shows τf data of several main stream LTCC dielectrics. It is expected that for integrated RF circuits using any of these LTCCs, the impact of τf will be reflected in a drift of resonant frequencies of embedded resonators over the device's operating temperatures. This drift results in extra design limitation and/or performance compromise.
Therefore, a need remains for an LTCC having localized temperature stability.